Radio frequency devices and circuits are used in a variety of applications, for example in communication devices. In such radio frequency (RF) devices, impedance matching networks and/or filters are frequently used which may be implemented with so-called lumped elements, i.e. discrete inductors, capacitors and other passive elements.
Such filters are for example used to remove spurious components such as harmonics of a base frequency, e.g. a carrier frequency, and intermodulation products from an RF signal. For example, modern RF power amplifiers may use high speed semiconductors which generate such spurious components in addition to a wanted carrier signal.
For example, in order to comply with regulatory standards, a level of unwanted emissions due to such spurious components originating from RF transmitters must be reduced below a certain threshold. Such thresholds may for example be specified in regional or country specific regulatory standards.
The filter transfer function of such a filter, i.e. the attenuation of the filter over frequency, depends on frequency responses of individual components, in particular inductors and capacitors.
Inductors are usually manufactured as coils. There are several different coil manufacturing technologies, but all are facing a common problem—with increasing frequency of signals applied to the inductors the quality factor (Q-factor) of the coil decreases, and, above a certain critical frequency, the inductance value of the inductor decreases abruptly. Such a degradation of inductance directly affects the effectiveness of filters and impedance matching networks using such inductors, which in some cases may lead to an insufficient attenuation of high order harmonics and other spurious effects of a RF signal. Such problems have become more relevant in recent years as frequencies of RF circuits increased, for example into the Gigahertz (GHz) range.
As a characteristic of discrete elements like inductors, resistors and capacitors usually data about the components behavior over a frequency range is given, specifying the so-called equivalent series resonant frequency (SRF). For a given type of coils and technology, the SRF value increases when decreasing nominal inductance value. A significant degradation of the Q-factor and decrease of the inductance as mentioned above may take place even before reaching the SRF, which limits the inductor effectiveness for rejection of high order harmonics in filters and matching networks.
Therefore, it may be desirable to provide one or more RF filters and/or impedance matching networks that may reduce or mitigate the above-mentioned problems.